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Thesis Format

Monograph

Degree

Doctor of Philosophy

Program

Chemical and Biochemical Engineering

Supervisor

de Lasa Hugo

Abstract

A typical FCC unit involves the transport and rapid catalytic reaction of chemical species using 60-70 micron fluidizable catalyst particles. In FCC, hydrocarbon species evolve in the gas-phase are adsorbed on, and then react with the catalyst particles. In this case, large molecular weight hydrocarbons (vacuum gas oil) are converted into lighter products (gasoline). FCC also yields undesirable products such as light gases and coke. Coke promotes catalyst activity decay and as result, is detrimental to catalyst performance. Given the significance of coke as a catalyst decay agent in FCC, it is the objective of this PhD research to study catalyst deactivation by coke.

To accomplish this, three different Y-zeolite FCC catalysts, designated as CAT-A, CAT-B and CAT-C were employed in the present PhD study. Catalyst samples studied were characterized in terms of Crystallinity, Total Acidity, Specific Surface Area (SSA), Temperature Programmed Ammonia Desorption (NH3-TPD) and Pyridine Chemisorption.

Catalytic cracking runs were carried out in a CREC Riser Simulator using a model hydrocarbon species (1,3,5-TIPB) as a hydrocarbon feedstock. This bench-scale mini-fluidized batch unit mimics the operating conditions of large-scale FCC units. Temperatures within the 510°C-550°C range and times ranging from 3s-7s were selected for catalyst evaluation. For every experiment, 0.2g of 1,3,5-TIPB was contacted with a catalyst amount ranging from 0.12g to 1g. This was done to achieve a C/O ratio in the range of 0.6 to 5.

Results obtained showed a consistent 1,3,5-TIPB conversion pattern for the three catalysts studied: increasing first, stabilizing later, and finally decreasing modestly. In spite of this, coke formation and undesirable benzene selectivity always rose. On this basis, a mechanism involving both single catalyst sites for cracking and two sites for coke formation was considered. In this respect, coke formation was postulated as an additive process involving coke precursor species, which are either adsorbed on two sites located in the same catalyst particle or adsorbed in two close sites in different catalyst particles.

Summary for Lay Audience

Fluidized Catalytic Cracking (FCC) involves a rapid catalytic reaction and the transport of chemical species between two phases: a) hydrocarbons as gas phase species and b) particles as a discrete solid phase. During this process, heavy molecules (gas oil) are converted into a lighter product (gasoline). However, this approach is accompanied with the undesirable formation of C1-C5 light gases and coke on the catalyst surface. Coke leads to catalyst decay and as result, is detrimental to catalyst performance. Given the significance of coke as a catalyst decay agent, the present PhD program is devoted to establishing a catalyst decay model suitable for the simulation of FCC in large scale riser and downer units.

To accomplish this, the present study pays special attention to various FCC operational parameters affecting coke formation and catalyst decay. To address these issues, runs were developed in a CREC fluidized Riser Simulator by varying: a) weight of the catalyst: 0.12g to 1g, b) Catalyst/ Oil ration (C/O ratio): 0.68 to 5, c) temperature: 510-550°C, d) contact time: 3 s to 7 s. The selected catalyst was an ECat FCC catalyst samples and the feedstock used was 1,3,5 tri-iso-propyl-benzene (1,3,5 TIPB).

Experiments findings in conjunction with advanced surface science techniques, allowed one to illustrate the influence of increasing the C/O ratio on 1,3,5-TIPB conversion, coke formation, and product selectivity. It was observed that a proper description of coke formation (e.g. Coke selectivity) and catalyst activity decay is required for an effective counting of Catalytic cracking with an ample range of C/O ratios (0.6–5 g-oil/g-cat).

Thus, it is anticipated that the postulated catalytic cracking reaction network influenced by catalyst density, affects both catalyst coking and deactivation, leads to an optimum C/O ratio, to accomplish maximum feedstock conversion, controlled coke-on-catalyst and gasoline benzene content. This is equivalent to a careful selection of both catalyst mass flow and hydrocarbon mass flow in large-scale risers or downers.

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